The major object of this project is to uncover enzymatic steps involved in various genetic rearrangement reactions and study the mechanism of action of the enzymes involved. The reactions studied include: the site specific recombination of bacteriophage lambda, resolution of the intermediate of general genetic recombination, and transposition-replication reaction of bacteripphage Mu. The most recent developments include the finding that endonuclease VII coded by bacteriophage T4 can resolve the Holliday structure, a typical general recombination intermediate, at high efficiency. Endonuclease VII and DNA ligase can cut and reseal the Holliday structure to restore normal DNA structure. This is the first example of the enzymatic step involved in this process.